I've had this cactus for about 6 months, and I can't figure out how to make him happy. I've tried watering more (he especially didn't like that) and I've tried watering less (better, but still not good). The first picture shows him when I first got him, he's the one in the red pot, second from the left. The second image is him now, much less healthy but not dead. Please help me identify him so I can learn what he needs and take better care of him!

It looks like Eve's Needle (Austrocylindropuntia subulata). When you first got it, it was in bad shape due to lack of sufficient light. Now, its rotting - probably a combination of overwatering, still insufficient light and cool temperatures. I'm not sure you can save it. Sorry!

Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost

I agree with Daisy that it arrived already weak and etiolated. It was started with a piece grown in strong light (the fat part at the base) but then allowed to stretch for quite a while afterwards (the skinny part above that). Even if you don't lose the plant to its current malaise, the etiolation is irreversible and the plant will be weak where the stem is skinny... meaning it won't get much bigger before it falls over under its own weight. Not to paint a dim picture, but that's usually what happens when cacti etiolate for a while.

What orientation does that window face? Do the plants "see" the sun during the day?

The two plants to the right of the cactus in question also look like they are not getting enough light. Compare the prickly pear pads with the yellow paint (the lower, original pads) with the white ones that have grown above them since. That difference in width is due to a drop in available light. The cactus to the right of it looks like it's growing a sort of nubbin on top which is stretched stem as well. That's what makes me think maybe it's a north facing window or something. Any brighter options?

Hello Maggie, it does seem to look like Austrocylindropuntia subulata. If it were mine, I would try to improve the soil. make it grittier. Ambient temperature is dependent on your growing location, so try to give it as much light and warmth.

By grittier media, my Eve's needle soil looks like this, I add lots of pumice into the soil and top dress it with a combination of more pumice and chicken grit so the base of the plant does not stay too moist. At times, roots gets too soaked, so it rots and plant is unable to take in moisture, so above soil line plant is looking too dehydrated being unable to drink. But if media is well draining and gets just enough moisture and good airflow at soil level, then the roots will thrive better, able to drink up and improve overall condition of the plant. Winter time is a bit dicey time for most succulents, most are either dormant or in slow down mode in their growth cycle. so lessening of watering frequency is important till temps go much warmer. Most cacti go on dormant mode during winter time, so it follows less to no watering till mid Spring returns.

Cutting the top off at the fat part is an option and would immediately answer the question of where the rot is and how far it has spread. Repotting is not a good idea at this point.

Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost